This invention relates to improvements to steerable downhole tools and particularly, although not exclusively relates to a device for locking and unlocking an asymmetrical offset sleeve relative to a drill string which rotates within it.
It is known to provide a steering device on the lower end of a drill string in order to steer the borehole away from the vertical. In certain circumstances, it is desirable to steer the drill bit in a short radius curve, in order to avoid certain rock structures or to tap into or drain smaller pockets of oil or gas. Many systems have been proposed for short radius curve drilling. One of these utilises a mud rotor to rotate a drill bit. The drill bit is tilted relative to the wellbore centreline, so that it drills a curved path. The rotational orientation of the motor housing in the borehole determines the direction of the curve of the borehole, so some means must be provided in this system to keep the motor housing oriented while drilling.
An alternative system for short radius curve drilling comprises what is known as a “constrained-rotary” drilling system. This system employs a flexible drive shaft which rotates inside an articulated non-rotating housing. A “curve guide” made of resilient material acts as a spring to apply a side force to the bit and thereby to cause the bit to drill a curved path.
A further system for short radius curve drilling comprises the “rotary-guided” system, in which a flexible drill collar is oriented by specialist downhole equipment. In this system, the flexible joint is provided in the drill string towards it lower end and the flexible joint is pushed towards one side of the hole to tilt the bit. The pushing force can be provided by a standard mule-shore sub for gyro orienting and a non-magnetic mule-shoe sub for magnetic orienting. This system is generally considered cheaper than the above-described methods. However, there are significant disadvantages in the systems currently available. These are:
1. The orientation equipment is not sufficiently stable and can therefore rotate slightly with the drill string thereby causing the borehole to veer off from its desired direction.
2. If the orientation equipment loses its grip significantly it can rotate around the bore hole in an uncontrolled fashion, gouging out the sides of the borehole.
3. No satisfactory means has been devised for biasing the flexible joint into a straight orientation, so that the assembly does not necessarily return to straight drilling, if the side force on the flexible joint provided by the orientation equipment is removed.
The prior systems have relied on rotary seals to seal the interior of the drill string relative to the exterior of the drill string. These rotary seals have caused maintenance and reliability problems and it would be preferably if they could be avoided.
In the prior art devices, means must be provided to latch and unlatch the orientation equipment relative to the rotating drill string. As the drill string can be rotating at speeds of from 100 to 300 rpm, and as the torque on the drill string as the drill bit advances can be enormous, reliable direct latching and unlatching of the orientation equipment relative to the drill string is difficult to achieve.
The various aspects of the present invention have been developed with these disadvantages in mind.